Direct injection gasoline engines offer improved efficiency because fuel injected directly into a cylinder can reduce cylinder charge temperature. As a result, additional air may enter a cylinder as compared to an equivalent cylinder that has port injected fuel. Consequently, engine power and efficiency may be improved. In addition, direct injection gasoline engines may exhibit improved transient fuel control because there is less tendency for fuel to collect at a cylinder intake port of a direct injection engine than for a port fuel injection engine. However, direct injection engines may generate soot at higher engine speed and load conditions because there is less time available to atomize fuel in the cylinder. As a result, it may be useful to incorporate a particulate filter in the exhaust system of a direct injection engine. Gasoline engines include those engines fueled by pure gasoline or mixtures of gasoline and other fuels such as alcohols. Further, other fuels used in spark ignited engines are also included such as liquid propane gas (LPG) or compressed natural gas (CNG).
In U.S. Patent Application 2009/0193796 a system for treating exhaust gases of a gasoline engine is presented. In several embodiments, a three-way catalyst is followed by a particulate filter. The particulate filter may be coated with various combinations of platinum, palladium, and rhodium. The coated particulate filter may assist in the oxidation of soot that is held by the particulate filter. It may be beneficial to filter gasoline engine emissions with a particulate filter, but over time, a particulate filter may accumulate an amount of soot to the extent that it reduces engine efficiency by increasing backpressure in the exhaust system. The reference appears to provide little direction as to how to remove soot from a particulate filter. Therefore, the system described in the reference may cause engine performance to degrade over time. In addition, the three-way catalysts described in the reference operate at higher efficiencies when gases entering the three-way catalyst are near stoichiometric conditions. However, there may be some engine operating conditions where the temperature of the particulate filter is less than desired to achieve a desired rate of soot oxidation. The reference appears to offer little direction for overcoming low particulate filter temperatures.
The inventors herein have developed a method for controlling a spark ignited engine having a particulate filter, comprising: retarding a spark angle at which spark is delivered to at least one cylinder of a spark ignited engine when an amount of soot held by a particulate filter exceeds a threshold and when engine load is less than a threshold level.
Oxidation of soot held by a particulate filter can be improved when the temperature of a particulate filter is regulated by retarding spark timing from minimum spark for best engine torque (MBT). For example, when an engine is idling at a low speed and low load, the engine may not generate sufficient heat to achieve a desired rate of soot oxidation for soot held by a particulate filter. However, in one embodiment of the present description, the crankshaft angle at which spark is delivered to a cylinder can be retarded so that the cylinder air-fuel mixture combusts later in the cylinder cycle, thereby reducing engine work and increasing an amount of heat discharged from the cylinder to the exhaust system. In this way, engine spark can be adjusted so that additional heat is provided to a particulate filter in the engine exhaust system. Thus, the particulate filter temperature can be increased to improve a rate of soot oxidation.
The present description may provide several advantages. Specifically, the description provides a method for increasing a rate of soot oxidation for soot held by a particulate filter. Further, the present method is responsive to driver torque demand so that engine torque increases as driver demand torque increases. In addition, the present method can reduce the time to regenerate a particulate filter because regeneration may be possible even during low engine loads.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.